A remote radio head (RRH) in a telecommunications system may use multiple-Input Multiple-Output (MIMO) technology in which multiple transmitters and receivers can be used to transfer more data at the same time. MIMO can exploit multipath propagation to enable more than one data signal to be simultaneously sent and received over the same radio channel.
In some instances, a butler matrix may be used to provide signals to antennae for transmission. A typical Butler matrix is made of a variety of quadrature hybrids and fixed phased shifters and is able to form a group of multiple beams fixed in space. Depending on whether transmitting or receiving, a Butler matrix connects each radio frequency (RF) input to the matrix to a unique beam output, or connects each RF output to the matrix to a unique beam input.
Typically, Butler matrices are used in to enable good behaviour in WCDMA systems. This is because, effectively, Butler matrices equalize the power between each transmission antenna port, which is needed for WCDMA to work efficiently. This is, however, not the case for LTE systems in which closed loop transmitted information can be correlated. This can lead to an imbalance in the power distributed between antenna ports, affecting power amplifier life duration.